The invention relates to extraction of precious metals, including gold and silver, by a leaching process from minerals that are difficult to treat by the cyanide process, and especially relates to overcoming problems associated with metallic iron that is present in a copper-ammonium thiosulfate lixiviating solution in a large scale processing plant.
Extraction of precious metals by lixiviation commonly is performed by using cyanide solutions, mainly sodium cyanide. Because cyanides are so highly toxic and cause substantial environmental problems, the use of cyanides is now falling into disfavor. Moreover, cyanides are costly materials. This makes their use economically disadvantageous. Furthermore, the use of cyanide solutions is at best difficult, and sometimes is impossible for some ores, especially those containing copper and/or manganese, because the latter materials easily contaminate the cyanide. Such materials as copper and manganese are frequently present in the ore to such an extent that high reagent loss is experienced, along with poor recoveries of the precious metals.
With respect to the last mentioned problem, there are many difficult-to-treat ores in existence which contain manganese, copper oxides and significant quantities of silver and/or gold. It would be very desirable to extract precious metals from such difficult-to-treat ores, if a suitable and sufficiently inexpensive technique were known for such recovery. However, present techniques simply are not adequate, so these difficult-to-treat ores remain an untapped mineral resource. One such source of difficult-to-treat ores containing precious metals is tailings from previously processed ores that were subjected to prior inefficient extraction processes, such as cyanide processes.
Copper sulfide containing ores such as calcocite and chalcopyrite often contain small quantities of gold and silver which it is desirable to recover. Although the problem of recovering such precious metals, in addition to recovering the copper, has received considerable attention, much of the work carried out in this connection insofar as commercial processing is concerned has involved the recovery of precious metals using pyrometallurgical processes for the recovery of copper.
One attempt to solve the above-identified problems is disclosed in the Genik-Sas-Berekowski et al. U.S. Pat. No. 4,070,182. This patent proposes the use of ammonium thiosulfate as a secondary leach for recovery of silver and gold, in conjunction with a hydrometallurgical process for the recovery of copper from the copper-bearing sulfidic ore. FIG. 3 of that patent shows a flow diagram for the extraction of precious metals from chalcopyrite concentrate before the main leaching step for extraction of copper. However, U.S. Pat. No. 4,070,182 appears to provide no suggestion as to how to maintain the thiosulfate radical stable, and it does not even appear to recognize the problem of thiosulfate instability. A time-related instability causing loss of recovery is mentioned but no reason therefore is suggested, nor is any solution proposed. The treatment of raw ores generally requires more time for a satisfactory recovery than is allowed in treating the sulfidic concentrates or residues Cs described in U.S. Pat. No. 4,070,182. That patent does not clearly teach the necessity of maintaining an alkaline pH in the thiosulfate leach liquor when starting with a raw ore, although the need for an alkaline pH is mentioned in conjunction with thiosulfate extraction following a copper recovery leach. Furthermore, the foregoing patent provides no guidance with respect to the extraction of precious metals from difficult, raw untreated ores, and more importantly, ores containing manganese.
U.S. Pat. Nos. 4,269,622 and 4,369,061 by Kerley, Jr. go further than U.S. Pat. No. 4,070,182 by describing a process in which the difficult-to-treat ores are treated by lixiviation in ammonium thiosulfate solutions containing copper with at least a trace of sulfite ions to extract gold and silver. After the lixiviation has been completed, recovery of the precious metals from the leach liquor is carried out using techniques that are conventional for recovering precious metals from cyanide solutions, such as by use of metallic zinc, iron or copper, by electrolysis, or by the addition of soluble sulfides to recover a sulfide precipitate. The stripped ammonium thiosulfate solution is then rejuvenated and can be recycled for re-use. The Kerley, Jr. process is described as being advantageous for recovery of gold and silver from difficult-to-treat ores such as those contaminated by copper and/or manganese. The Kerley, Jr. patents teach that some copper must be present for good recovery and also teach that it is desirable to maintain the pH of the leach solution in the range from at least 7.5, and preferably 8. Sulfite ions are provided by the Kerley references by adding ammonium sulfite or ammonium bisulfite to the leaching solution to inhibit decomposition of the thiosulfate. The lixiviation is taught to be preferably carried out at a temperature of 50.degree. to 60.degree. Centigrade. Kerley, Jr. teaches that temperatures below 40.degree. Centigrade adversely effect the speed of the process. On the basis of the favorable results obtained in laboratory experiments using the process of the Kerley, Jr. U.S. Pat. No. 4,269,622, a very large amount of capital was spent building a large plant in Mexico to carry out the invention disclosed in the Kerley patent. Unfortunately, although the process worked well in a laboratory environment, we were unable to ever get the process to work in the above mentioned plant, despite extensive consultation with the inventor, Mr. Kerley.
Subsequent to the failure of the process disclosed in the Kerley, Jr. patent to operate in the constructed plant, we, with the expenditure of a large amount of money, worked for over two years modifying the process of U.S. Pat. No. 4,269,622, attempting to develop a technique that would economically leach gold and silver from available difficult-to-treat ores. Only after many man-hours of effort and after expenditure of a great deal of money were we able to develop the modified process of the present invention so that it would be operative in a large plant.